1. Field of the Invention
This invention relates generally to the fields of optics and lasers, and more particularly to protecting sensitive optical elements from alteration or damage due to exposure to trace atmospheric species during shipping, storage or use.
2. Description of the Related Art
Many optics and laser systems employ sensitive optical materials that may be damaged by exposure to trace elements or components in the surrounding environment. In particular, certain materials that are of great interest because of their unique optical properties are prone to interact with a surrounding atmosphere to an extent that is sufficient to change the material""s physical structure, resulting in subsequent degradations in the material""s performance. This presents a problem for in situ protection of these materials in both everyday use, shipping and storage. For some materials, even ambient levels of water vapor of a few percent by mass may pose a threat, while for other materials, certain chemical species in the surrounding environment such as organic molecules may be an issue.
Borate crystals are an example of one class of materials that may suffer deterioration in performance upon mere exposure to ambient environment, such as air. This is because the crystals are hygroscopic, and can chemically react with absorbed water molecules. Such reactions can cause undesirable alterations in the crystals"" optical and physical properties. Examples of borate crystals include BBO (xcex2-BaB2O4), LBO (LiB3O4) and CLBO (CsLiB6O10), all of which found considerable use in the nonlinear conversion of light from infrared and visible lasers into the UV spectral range. These crystals differ by their optical and physical properties including the degree to which they are innately prone to absorbing water vapor from a surrounding environment. Among these crystals, CLBO has become the crystal of choice for harmonic conversion to wavelengths shorter than 300 nm because of fortuitous combination of optical properties and nonlinear parameters. It is currently considered to be the most suitable material for fourth and fifth harmonic generation of laser light from infrared Nd-doped lasers at high powers. CLBO is however, also particularly hygroscopic. The literature on the deleterious effects of moisture on CLBO includes the article by Taguchi et al in Advanced Solid State Lasers, C. R. Pollock and W. R. Bosenberg, eds. OSA Vol. 10, pp.19 (1997), where correlations were described between the rate of surface hydration and induced cracking and refractive index changes. Such humidity dependent characteristics are generally considered to be highly detrimental to reliable and long term use and operation of CLBO in high power laser systems.
Other examples of known hygroscopic crystals are the so-called ADP-isomorphs, such as KD2PO4 (KD*P), NH4D2PO$ (AD*P) and CsD2AsO4(CD*A). These crystals are commonly used in electro-optic light modulators or in non-linear frequency conversion. Among this class of materials, CD*A is known to be particularly sensitive to humidity levels.
Several techniques have been employed to protect strongly hygroscopic materials such as CLBO, CD*A and the like, from the deleterious effects associated with water vapor absorption. They include special coatings, hermetic sealing with gas purging and operation at elevated temperatures.
Anti-reflective coatings are traditionally applied to crystalline materials used in laser systems to prevent Fresnel losses. Yet, coatings are also useful as a barrier to prevent water vapor or oxygen molecules from permeating the crystal, and causing disadvantageous changes in the physical structure and attendant thermal and optical properties thereof. Use of protective films on crystals subjected to UV radiation was described, for example, in U.S. Pat. No. 5,862,163. Such coatings were useful for crystals such as BBO, which is not highly hygroscopic or reactive. Coatings have proven more problematic for more reactive crystals such as CD*A and CLBO, because the process of applying the coatings can itself precipitate further damage mechanisms at the interface between the crystal and the coatings. Furthermore, coatings become increasingly susceptible to damage as the wavelength of the light becomes shorter, an acute problem for CLBO which is most often used to convert light into the deep UV. For highly sensitive materials, in particular, there is often a negative trade-off between the requirement that the film be thick enough to prevent water permeability, yet be thin enough to avoid damage. Thus, in and of themselves, coatings may not be sufficient to provide the needed protection from moisture and other potentially harmful gas species.
Another protective measure involves use of clean room preparation and assembly techniques to seal the enclosure containing the sensitive material followed by purging with a high purity gas that is inert with respect to chemical interactions that may change the material""s physical or optical properties. Such sealing and purging methods using most commonly Nitrogen or an inert element as a purge gas, are well known in the art of laser fabrication, assembly and maintenance, and purging is often used as a standard procedure for example, during field service, following the replacement or repair of optical elements contained within the laser. However, when the optical element is extremely fragile and is especially sensitive to contamination even by trace gas components, special precautions must be taken to provide sufficient protection against ambient environment. For example, the cell containing the material may be hermetically sealed, to prevent any potential for leakage or contamination, in which case a single charge of gas can be used. Alternatively, the purge gas may flow continuously or at intermittent intervals, in which case the chamber containing the material must still be tightly sealed against the external environment, and a complete gas pumping and purification system must be further provided as part of the assembly, with a ready supply of purge gas maintained at all times.
While effective in providing a degree of protection against contamination, techniques of purging and tight sealing present a number of significant practical disadvantages. In particular, tight, or, in the extreme case, hermetic sealing of the chamber does not allow for any ready access to the optical material for the purposes of adjustment, inspection or replacement. In addition, if windows that are transparent to deep UV light must be provided as part of the sealed cell, as is the case for example, for CLBO used in a harmonic conversion module, there is a substantial risk of damage to the window material, especially at higher powers. Combined with the limited accessibility, the need to design so as to avoid damage to windows limits the design flexibility of the entire system containing the material.
Complete gas sealing also has the complication that altitude or other atmospheric changes as may be encountered during shipping can produce undesirable forces on the mechanical system containing the cell, leading to potential misalignments which are not readily corrected. Considerable cost, bulk and complexity are also added to the system, whether a hermetically sealed cell is utilized or a complete purge system is included, even as the overall reliability and longevity of the entire system may be compromised by a potential for catastrophic failure should the purge unexpectedly fail, or the gas charge dissipate. This failure mode is especially problematic when shipping a device in which CLBO or a similarly sensitive material is a component. By land, sea or air, practicality demands that devices be unattended for extended times.
Whenever the purge is not operating, or and/or the tight sealing is compromised due, for example, to extreme temperature and pressure variations, the crystal may be left insufficiently protected and may become momentarily exposed to undesirable humidity or other contaminant levels. Should irreversible damage occur to the sensitive material in the field, the entire sealed cell or chamber assembly must be replaced, thereby increasing the life cycle costs of the system.
In the case of CLBO it was shown that temperature annealing of the crystal and operation at elevated temperatures can significantly reduce index distortion and other optical damage effects. It is customary to place the CLBO or similar sensitive material in an oven. The oven mitigates against absorption of gas species because the heated material acts very rapidly to eliminate condensation by thermalizing any contaminant gas molecules that are adsorbed on the material""s surface. When contaminant molecules comprise water vapor, maintaining the crystal at elevated, constant temperature can reduce surface hydration and subsequent physical alterations. Practical implementation of this technique requires avoidance of heating and cooling cycles, necessitating continuous operation of the oven surrounding the sensitive material, even during storage and transportation.
Techniques for controlling the temperature of a nonlinear crystal are disclosed in U.S. Pat. No. 6,002,697, where provisions are included for purging a tightly or hermetically sealed housing containing the crystal so as to remove any moisture from the surrounding environment. This method requires a continuous actively controlled thermal and physical environment, as the oven(s) have to be continually supplied with power, and a complete purge assembly must also be provided as was described above. Electrical power is typically provided by bulky, heavy batteries or by access to an existing electrical grid. During shipping, there is lack of access to the electrical grid whereas the bulk and weight of batteries as well as security issues can be problematic. Thus, failure of either the power supply and/or the purge system can have catastrophic effect on the crystal.
Solutions to the problem of in situ protection of contamination-sensitive optical materials are therefore desired that involve methods that do not inherently require exchanging mass with an external source (purging) to establish a uniform steady state with respect to chemical composition, or exchanging energy to establish a non-uniform steady state with respect to temperature (heating) within the enclosure. The related art suggests one passive technique described in U.S. Pat. No. 6,036,321 which teaches use of a desiccant material to absorb water vapor from a surrounding environment, thereby protecting the hygroscopic material. The specific teaching involves coupling a desiccant container to a volume containing the hygroscopic material with a tube or a duct.
While such a structural arrangement has the dual advantages of i) isolating the hygroscopic material from any potentially deleterious effects of exposure to the desiccant (for example, out-gassing or aerosol generation); and ii) isolating the desiccant from any potential damage from radiation that may illuminate the volume containing the hygroscopic material, the tube/duct inherently restricts the flux of contaminants. It is therefore applicable to comparatively less hygroscopic materials such as LBO, KDP and KD*P. While water molecules will eventually diffuse into the volume containing the desiccant, resulting in moisture reduction, the time required to remove all traces of water vapor from volume surrounding the crystal may be too long for comparatively sensitive materials like CLBO or CD*A, especially during shipping when temperature and pressure can vary rapidly over a wide range.
There is therefore a need for a passive method and apparatus for protecting sensitive materials that is compatible with rapid removal of trace components such as water from the surrounding ambient environment. There is a further need for a method and apparatus for protecting sensitive materials that can accommodate materials with different rates of absorption even under conditions that include wide temperature and pressure extremes.
Accordingly, an object of the present invention is to provide a passive method and apparatus for in situ protection of sensitive optical materials.
Another object of the present invention is to provide a method and apparatus for in situ protection of optical materials that is compatible with rapid removal of trace components such as water from the surrounding ambient environment.
A further object of the present invention is to provide a method and apparatus for in situ protection of optical materials that can accommodate materials with different rates of absorption over wide temperature and pressure ranges.
These and other objects of the present invention are provided in an optics housing with an enclosure that has an interior volume and is configured to be substantially sealed against an external atmosphere. An optical element is positioned in the interior volume. The optical element includes a material having at least one physical characteristic that varies with exposure to at least one constituent of the external atmosphere. A container is coupled to the housing and includes a gas-permeable surface. A sink material is disposed within the container. The sink material sorbs at least one gas species.
In another embodiment of the present invention, an optics housing includes an enclosure with an interior volume. The enclosure is adaptable to be substantially sealed against an external atmosphere. An optical element is disposed within the interior volume. The optical element contains material with at least one physical characteristic that varies with exposure to at least one constituent of the external atmosphere. A container is coupled to the enclosure. The container includes a gas-permeable surface area with a ratio of no less than 0.1 to a surface of the container. A sink material is disposed within the container. The sink material has a characteristic of spontaneously sorbing at least one gas species.
In another embodiment of the present invention, an optics housing includes an enclosure with an interior volume area. The enclosure is adaptable to be substantially seals against an external atmosphere. An optical element is disposed within the interior volume. The optical element contains material having at least one physical characteristic that varies with exposure to at least one constituent of the external atmosphere. A container is coupled to the housing and has a gas-permeable surface and a gas impermeable access port. At least a portion of the container forms a protuberance that extends into the interior volume of the housing. The gas impermeable access port is a portion of an optics housing exterior. A sink material is disposed within the container. The sink material has a characteristic of sorbing at least one gas species.
In another embodiment of the present invention, a laser system includes an enclosure with and interior volume and an interior surface area. The enclosure is adaptable to be substantially sealed against an external atmosphere. A laser is position in the interior volume. An optical element is disposed within the interior volume. The optical element contains material with at least one physical characteristic that varies with exposure to at least one constituent of the external atmosphere. A sink material is position on the interior volume. The sink material has a characteristic of spontaneously absorbing of at least one gas species.
In another embodiment of the present invention, an optics housing an enclosure with an interior volume and an interior surface area. The enclosure is configured to be substantially sealed against an external atmosphere. An optical element is disposed within the interior volume. The optical element contains material having at least one physical characteristic that varies with exposure to at least one constituent of the external atmosphere. A sink material is positioned in the interior volume. The sink material has a characteristic of spontaneously absorbing of at least one gas species.
In another embodiment of the present invention, a laser system includes an enclosure with an interior volume and an interior surface area. The enclosure is adaptable to be substantially sealed against an external atmosphere. A laser is positioned in the interior volume. An optical element is disposed within the interior volume. The optical element contains material with at least one physical characteristic that varies with exposure to at least one constituent of the external atmosphere. A sink material is positioned in the interior volume. The sink material has a characteristic of spontaneously absorbing of at least one gas species.
In another embodiment of the present invention, a method for protecting sensitive optical elements in situ provides an enclosure with an interior volume. The enclosure is adaptable to be substantially sealed against an external atmosphere. An optical element is disposed within the interior volume. The optical element includes material having at least one physical or optical characteristic that varies with exposure to at least one constituent of the external atmosphere. A sink material is provided and coupled to the enclosure. At least one gas species is trapped in the sink material by a sorption process. The at least one gas species includes at least one constituent of the external atmosphere with which the at least one physical or optical characteristic of the optical element varies.